In the operation of a turbine, for example a steam turbine, preferably for generation of electric power, great demands are placed on the control of the turbine, above all concerning its speed at varying loads and the control means available is then above all the control valve of the turbine by which the flow to the turbine can be regulated.
This regulation must then take all operational conditions into account, normal, slower variations in the load as well as suddenly arising situations such as loss of load, short-circuits of the electric supply networks and line breakdowns.
In order to cope with the demands for the control of the control valve, which is of considerable size in large turbines, said control valve is usually provided with a hydraulic servo motor, for example a piston moving in a hydraulic cylinder, for opening of the control valve while the closing is performed by means of a strong spring after the servo motor has been relieved of its hydraulic pressure.
The control of the servo motor is performed by means of a control valve unit by which the servo motor is connected to a hydraulic feeding source for opening of the control valve, or to an outlet when the control valve is to be closed.
This control valve unit is controlled from the control system of the turbine which, for normal operational conditions, comprises a turbine regulator for normal operation, normal starting and normal stopping. Parallel thereto is a release system for rapid closing of the control valve in the event of an emergency or serious faults such as line break-downs.
Whereas normal opening and closing operations of the control valve require a time elapse of several seconds and great precision, rapid closing should be effected in about 0.1 seconds or shorter. To be able to fulfill such diverse requirements with one and the same control valve unit it has hitherto been normal to arrange several regulating steps in succession with hydraulic amplifier steps therebetween, which is both expensive and complicated.